The invention relates to an inductive sensor arrangement for detecting a ferrous object buried in a surrounding medium comprising a field coil pair for generating a penetrating alternating magnetic flux field in said medium, and a sense coil for sensing magnetic flux field disturbances caused, as the case may be, by said ferrous object.
The invention further relates to a method and its application in a hand-held machine tool for detecting a ferrous object hidden in a surrounding medium by use of the inductive sensor arrangement according to the present invention.
Metal detectors usually work by measuring the change of a chosen parameter as an operator sweeps a sensing head across a surface of interest of a medium in which a disturbing piece of a metal may be hidden. The parameter could be capacitance, inductance or any other physical parameter that would allow a distinction of one material from another.
The requirement may also be to find reinforcing bars (xe2x80x9crebarsxe2x80x9d), usually consisting of ferrous materials, embedded in media-like concrete, brick, plaster and the like. There are detectors on the market that can fulfill this requirement but the accurate detectors must be swept across the surface of the medium, e.g. concrete. Through the xe2x80x9csweeping movementxe2x80x9d of the detector it is possible through the use of the received response signals to determine the position and the (length) direction of the hidden object, e.g. the rebar. The metal coverage area may either be determined manually or automatically via a rather complicated system. In the manual determination, it is the usual practice to manually mark the coverage and direction, of the rebar, on the surface of the medium. Needless to say that this manual scanning and determination requires not only time but also a specific skill and knowledge of the user or operator.
U.S. Pat. No. 5,729,143, which presently Applicant regards as the closest prior art, metal detector including a receive coil and a transmit coil arranged in parallel overlapping winding planes and connected in an inductive bridge. This is a typical example of a metal detector that needs specific skill and knowledge of the operator for interpreting the signal response. DE 196 48 833 A1 describes an alternative prior art device for detecting and identifying hidden objects like plastic mines in a ground. This device comprises two side-by-side arranged sensor coils that are operated at different excitation frequencies. Depending on various physical properties of the hidden object like electrical conductivity, permeability etc. the impedance of a receiver coil arranged in an overlapping configuration of said two sensor coils is modified differently depending on the respective material properties. Again, the scanning of a specific ground area and the interpretation of the receive signals requires experience and skill.
It is an object of the present invention to provide an inductive sensor arrangement and a method for detecting metal objects like rebars hidden in a medium.
More specifically, it is a further object of the invention to offer an inductive sensor arrangement and to provide a method for detecting metal objects hidden in a surrounding medium by a single position measurement or a sequence of measurements, in a single position of a sensor head, in order to evaluate the position, and the coverage of a respective metallic object relative to the measurement position.
A still further object of the invention is to provide a method for determining the depth of the metal object form the surface of the medium.
An inductive sensor arrangement in accordance with the invention comprises a field coil pair and a pair of sense coils, wherein two field coils of a field coil pair are adjacently positioned at a defined distance from each other with non-overlapping winding planes and are arranged essentially in the same geometric plane, and wherein one coil of the pair of sense coils is respectively mounted inside of each field coil in an orientation to the axis of each of said field coils such that essentially no voltage is induced in said sense coils, in an environment free of a ferrous object or material. Preferably, the axes of the said sense coils are arranged orthogonally with respect to the axes of each of the corresponding one of said field coils.
A method for detecting a ferrous object hidden in a surrounding medium by use of the inductive sensor arrangement according to the invention, comprising the steps of exciting the field coil pair with predefined current ramps sequentially supplied to the two field coils of one field coil pair to produce a changing magnetic flux, penetrating the medium with at least two magnetic field patterns originating from different physical positions, collecting of four distinct output voltages from the two sense coils of one sense coil pair, i.e.
a first output voltage from a first sense coil and a second output voltage from the other sense coil of said sense coil pair while the first field coil corresponding to the first sense coil is excitated by a first one of the predefined current ramps,
a third output voltage from the other sense coil and a fourth output voltage from the first sense coil of the sense coil pair while the other field coil assigned to the other sense coil is excitated by a subsequent one of said defined current ramps, and gaining a present or non-present criterion for a ferrous object by algorithmic processing of the four voltages.
Preferably, the algorithmic processing steps are performed sequentially with respect to a set of one voltage value, each of the four output voltages sensed by the sense coil pair during one excitation current ramp cycle being supplied to the field coil pair.
With the invention it becomes possible to accurately locate a metallic object, in particular a rebar, from a single point measurement. Accordingly, a sensor head and measuring unit according to the invention is simple to use, reliable and usable in confined spaces because of the one point measurement.